Avoid tmpfs inode exhaustion in scratch worktrees #231

Problem

Agent scratchpad worktrees under /tmp use a small tmpfs inode pool. A normal pnpm install can exhaust that pool before the dependency tree is complete, even when byte capacity remains.

Impact

Fresh JJ workspaces fail partway through setup and require a retry on a different location, wasting time and leaving an incomplete environment.

Reproduction

Run pnpm install in a scratchpad JJ workspace under /tmp with a large monorepo dependency tree. The install fails with ENOSPC after roughly 1.6k of 2k packages as the inode limit is reached.

Acceptance criteria

• Scratchpad guidance defaults dependency-bearing worktrees to a disk-backed path (for example ~/.cache/agent-ws) or performs a clear inode-capacity preflight before installation.
• The documented path prevents an inode-exhaustion retry for the existing shuv2code workspace setup.
• Add a focused guard or verification that reports the actionable path when the tmpfs limit is insufficient.